21681424 has 10 divisors (see below), whose sum is σ = 42007790. Its totient is φ = 10840704.

The previous prime is 21681421. The next prime is 21681449. The reversal of 21681424 is 42418612.

It can be written as a sum of positive squares in only one way, i.e., 17808400 + 3873024 = 4220^2 + 1968^2 .

It is a junction number, because it is equal to n+sod(n) for n = 21681392 and 21681401.

It is not an unprimeable number, because it can be changed into a prime (21681421) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 677529 + ... + 677560.

It is an arithmetic number, because the mean of its divisors is an integer number (4200779).

Almost surely, 2^21681424 is an apocalyptic number.

It is an amenable number.

21681424 is a deficient number, since it is larger than the sum of its proper divisors (20326366).

21681424 is a wasteful number, since it uses less digits than its factorization.

21681424 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1355097 (or 1355091 counting only the distinct ones).

The product of its digits is 3072, while the sum is 28.

The square root of 21681424 is about 4656.3316033118. The cubic root of 21681424 is about 278.8448326347.

The spelling of 21681424 in words is "twenty-one million, six hundred eighty-one thousand, four hundred twenty-four".